The present invention relates to an image reader for a facsimile apparatus or like image processing apparatus and, more particularly, to an image reader capable of preventing noise from being introduced in image signals and, thereby, improving the signal-to-noise (S/N) ratio by use of a store type amorphous silicon line image sensor.
Image reading means heretofore installed in an image processing apparatus is implemented with a line image sensor, as in an image reader section of a facsimile terminal. The line image sensor is constructed and arranged to photoelectrically transduce one line of images on a pixel basis. Generally, a line image sensor comprises a light-sensitive section where a plurality of light-sensitive cells such as photodiodes are arranged in an array, and a drive section adapted to sequentially select and deliver output signals of the light-sensitive cells. Recently, a so-called store type amorphous silicon line image sensor has been put to practical use which has as its light-sensitive cells photodiodes that are made of amorphous silicon. In an image reader of the kind using a store type amorphous silicon line image sensor, since the light-sensitive surface of each cell can be dimensioned as small as an actual pixel, images on a document need only to be focused in their actual size onto the line image sensor. This advantage leads to a small-size image reader construction.
A basic construction of such a store type amorphous silicon line image sensor is shown in FIG. 1.
In the illustrated image sensor, generally 10, a capacitance Cd represents a coupling capacitance of a photodiode, or light-sensitive cell, PD, while C.sub.L represents a capacitance developing in a wiring between the photodiode PD and a circuit to follow (e.g. amplifier). A resistor R is a current-limiting resistor adapted to read out an output. A source voltage V.sub.D is applied via the resistor A. A switch SW for charging the photodiode PD comprises a MOS (metal oxide semiconductor) switch or like semiconductor element.
Assume that the switch SW is turned on to charge the capacitances Cd and C.sub.L and then turned off to set up an image signal storing condition. In this condition, a photocurrent Ip complementary to a quantity of received light, i.e., a pixel luminance associated with a read image, develops in the photodiode PD to gradually discharge the capacitance Cd. When the switch SW is turned on again, the photodiode PD produces an output voltage Vout which based on charge conservation is expressed as: EQU Vout=V.sub.D -(Ip.multidot.T/(Cd+C.sub.L)) Eq. (1)
where T is the interval between consecutive turnons of the switch SW, or image information storing period.
Meanwhile, in the case where the whole charge stored in the capacitance Cd is discharged by the photocurrent Ip which has flown over the storing time T, the output voltage Vout of the photodiode, or saturation output Vsat, is produced by: EQU Vsat=C.sub.L .multidot.V.sub.D /(C.sub.L +Cd) Eq. (2)
Hence, the output Vout of the photodiode PD varies from the source voltage V.sub.D to Vsat complementarily to the photocurrent Ip which has flown over the storing time, i.e. luminance of the associated pixel. In this manner, image signals corresponding to pixel densities are provided.
The store type amorphous silicon line image sensor may be designed to read an A4 format document, which has a reading width of 216 millimeters, eight dots per millimeter by way of example. In this type of image sensor, 1728 photodiodes PD each having a light-sensitive area substantially equal in dimensions to a pixel are arranged at equal intervals in correspondence with the reading width; switches SW are connected in one-to-one correspondence and serially to the photodiodes PD. Where this type of line image sensor is driven as a single element, the capacitance C.sub.L increases to a significant level. The Eq. (2) teaches that an increase in the capacitance C.sub.L is reflected by a decrease in the level of the saturation output Vsat which in turn narrows the available dynamic range.
An implementation heretofore employed to preserve a desirable dynamic range consists in dividing the light-sensitive cells of a line image sensor into a plurality of blocks and driving them on a block basis. An example of such a prior art arrangement is shown in FIG. 2. In the illustrated example, photodiodes PD and switches SW are divided into n discrete blocks BL1 to BLn each having m photodiodes and m switches. For example, 1728 combinations of photodiode PD and switch SW are divided into twenty-seven blocks by sixty-fours. The outputs from the blocks BL1 to BLn are coupled to an amplifier AM via selection switches SL1 to SLn.
As represented by waveforms a to i in FIG. 3, the selection switches SL1, SL2, . . . , SLn, and the switches SW11 to SW1m, . . . , SWn1 to SWnm are operated by a controller, not shown, to produce one line of image signals Va. In FIG. 3, the logical "L" level represents the "on" state of each switch. First, after the selection switch SL1 associated with the block BL1 has been turned on, the switches SW11 to SW1m are sequentially turned on each for a charging period so as to apply output signals of the respective photodiodes PD to the amplifier AM. As the block BL1 is fully read out, the selection switch SL1 is turned off and, instead, the selection switch SL2 is turned on to read the next block BL2. Thereafter, the same procedure is sequentially repeated on the other blocks down to BLn.
The problem with the prior art arrangement as discussed above is that since the image signals Va are very weak signals such as on the order of 10 millivolts, they are noticeably effected by about several millivolts of switching noise NZ which is generated every time each of the switches SW is turned on and off (see waveform i of FIG. 3). The prior art image reader, therefore, accomplishes only a limited S/N ratio. In addition, switching noise entailed by the actions of the selection switches SL1 to SLn also has substantial influence on the image signals Va.